1. Field of the Invention
This invention relates to ionomer resin compositions based on ethylene copolymers which have high melt flow. They are ideally suited for use particularly as blending resins with other ionomers, to provide ionomer blends which also have high flow, yet have good resilience and durability. The compositions are readily processed to produce covers for golf balls. The resulting balls also have good resilience, and also have good durability. More particularly, the compositions contain a certain amount of ionomer prepared from ethylene carboxylic acid copolymers with a very high melt index, neutralized to a fairly high level.
2. Description of Related Art
Ethylene (meth)acrylic acid copolymers neutralized with metal ions are known as ionic copolymers or ionomers or ionomer resins. They are well known for use as covers for golf balls. Such resins are sold by E. I. dupont de Nemours and sold under the trade name of SURLYN. The properties of the ionomer are reflected to a considerable extent in the behavior of golf balls when the ionomer is used as cover material. The resilience of the golf ball is, for instance, dependent on the resilience of the cover material, and the durability of a golf ball on repeated impact reflects the toughness of the cover material. However, to obtain good properties, the melt index (MI) of the ionomer is generally so low that poor moldability of the material can be a problem. While low MI can give good properties, the poor moldability leads to molding inconsistency, and often the best possible properties are not realized.
Ethylene copolymer ionomers are made by neutralizing the corresponding ethylene/acid copolymer with materials which supply metal ions. The process of making ionomers was first disclosed in U.S. Pat. No. 3,264,272 (Rees). Neutralization causes ionic crosslinking to occur and at use temperatures (for golf balls this means ambient temperatures), this ionic crosslinking has a major beneficial effect on certain properties. Fortunately, at higher temperatures, for instance above 160 deg. C., the crosslinks become labile and the material can become fluid enough to process as a thermoplastic melt rather than remain intractable, as covalent crosslinked materials are. The crosslinking effect caused by the ions does not completely disappear at processing temperatures however, and the melt viscosity of these ion neutralized resins is still significantly higher than the parent un-neutralized resin. As the level of neutralization increases, viscosity increases many fold, and at levels above 90%, the resin can become almost intractable. Below about 20% generally insufficient property advantage compared with the base resin is achieved. For these reasons neutralization is typically in the range of 20 to 70%.
The melt viscosity, is normally quantified by Melt Index, (MI) which is a melt flow measurement and therefore has an inverse relation to viscosity. Higher MI, within limits, generally means better processability. On the other hand, very generally, lower MI, all else being equal, gives better mechanical properties. Typically ionomers for general use have an MI between 0.1 and 25. For golf balls, where premium properties are required, lower MI of below 3 is typically required.
In many linear polymers, MI for a given polymer chain composition is often a guide to mechanical properties, because it gives a guide to the length of the polymer chains, and this determines many strength, impact and toughness properties. If however the polymer is highly branched or partially crosslinked, MI is less of a guide to either the properties or to the length of the underlying polymer molecules, because MI is also dependent on the amount of branching or crosslinking. In the case of ionomers which remain partly crosslinked at temperatures where MI is measured, and which are effectively highly crosslinked at use temperatures, MI is not a unique guide to properties even for the same ionomer chemically. The properties of ionomers depend on the underlying molecular weight of the acid copolymer before ionic crosslinking and well as on the amount of crosslinking. The MI of an ionomer of given composition does not uniquely characterize that ionomer, since the same MI can be achieved either by highly neutralizing (i.e. crosslinking) a very high MI (lower molecular weight) acid copolymer, or neutralizing to a much lower degree, a low MI (i.e. higher molecular weight) acid copolymer. The resulting ionomers are structurally very different, because the underlying molecular weight of the acid copolymer remains an integral part of the ionomer's structure, as does the level of neutralization or ionic crosslinking. Thus the MI of the acid copolymer, not just the MI of the final ionomer, represents a key measure which defines the exact nature of the ionomer. Conventional wisdom holds that, for a given property level requirement, there is a clearly defined lower limit to the molecular weight of the underlying (non-crosslinked) acid copolymer, irrespective of the amount of neutralization--hence an upper limit to the MI of the acid copolymer from which the ionomer is made. This upper limit depends on the use, and properties required. For premium uses where excellent properties are required, a lower MI limit of the starting acid copolymer is necessary.
Rees discloses that the `base resin`, i.e., the acid copolymer, may have a melt index of from 0.1 to 1000, (using ASTM D-1238 at 190 deg. C.), but preferably from 1.0 to 100. Ionomers made from acid copolymers having a very high melt index of about 400 have been used for certain adhesives and binder applications, where mechanical property requirements are not as stringent as in those used for most other applications.
Neill et at. U.S. Pat. No. 4,774,290 discloses a method of making ionomers where the acid copolymers are neutralized with metal oxides using a polyethylene carrier resin to provide more uniform dispersion and reaction of the metal oxide. Composition variables are very broad. Thus, suitable starting MI of the acid copolymer is disclosed as being anywhere from 1 to 3000, preferably from 10 to 500, and the amount of carrier polyethylene from 0.5 to 100 parts per 100 parts of acid copolymer, (i.e., 50/50 polyethylene/ionomer). Final MI of the ionomers made varied from 0.87 to 14.87. An ionomer prepared from an acid copolymer of MI about 300, with about 1.6% polyethylene carrier resin, neutralized with magnesium oxide to an MI of 4.53 is exemplified. The polyethylene is described as increasing the strength and toughness of the final ionomer, as well as reducing opacity. A composition neutralized with about 3 weight percent magnesium oxide with 3% weight percent polyethylene carrier, and unspecified final MI was described as being suitable for articles including golf ball covers. Even very low levels of polyethylene however have been found to be detrimental to certain key properties for golf ball cover material.
For pure ionomers, free from other polymers, useful for golf ball covers, the upper MI limit of the precursor acid copolymer was generally assumed by the present inventors to be about 100, and higher MI base resins were never used. Recently, Yamada, U.S. Pat. No. 5,244,969 disclosed a higher limit of 150 for the MI of the acid copolymer base resin for ionomers specifically for golf ball covers. Yamada discloses that if the MI of the acid copolymer is above 150, the rebound resilience of his compositions is poor. Such resilience is a key property required in golf ball cover material.
When ionomers were prepared from acid copolymers with MI below 100 and neutralized to an MI of from about 0.4 to about 2.8, the ionomers were ideally suited, particularly in blends, to provide golf ball cover materials which allowed the balls to have premium properties--the desired high resilience, the durability, and the other characteristics sought after in golf balls. Ionomers with MIs much above 3.0 generally gave less than desirable properties.
Unfortunately, resins with low MIs, particularly about 3 or less do not lend themselves to as efficient and rapid processing to form golf ball covers as would be liked. This imposes a cost penalty both because of slower processing, and because of inconsistency in parts when molding a resin of insufficient fluidity. Typically the result is a high number of rejects. Thus, conventional wisdom imposed a window on pure ionomer compositions suitable for golf ball covers. The limits of this window were a starting acid copolymer MI no higher than 150 and preferably less than 100, and derived ionomer MI below 3.0 for properties. It is clearly possible, when starting with an acid copolymer having an MI of less than 150 to make an ionomer with higher MI for better processing, above 3 for instance, simply by neutralizing only to a relatively low level. However, high neutralization levels are considered critical in obtaining the beneficial properties of ionomer. Thus it appeared, one was forced into the best compromise between optimum flow and optimum properties.
Many other variations in the compositions of ionomers is known, and these variations have been used to optimize particular characteristics of golf balls when the ionomer material is used as its cover. For instance, the amount of acid and which acid, and the particular ion used to neutralize, as well as the amount of neutralization, affects the nature of the ionomer. Blending ionomers also has become common in production of such cover materials. Blends may be of different ionomers having different metal ions, different ionomers having different flexural modulus, or different ionomers having different acid level and/or type. Such blending has been found to provide a certain degree of synergism with respect to key properties important for golf ball covers, particularly resilience. Use of blending is now the norm for golf ball covers. When any new technology based on new composition or process variables is found, it is therefore common, or even the norm, to combine it with the various established blending and other property optimization technologies of the past.
With regard to the present problem, the problem of poor processability, there is a need for ionomer resins for use in golf ball cover materials, to be used alone, (but preferably as part of ionomer blends, as noted above), which have more desirable processibility, or impart better processability, without substantial loss of properties, and which lead to acceptable characteristics in golf balls when such materials are used as the cover, particularly durability and resilience characteristics.